This invention relates to magneto-optical recording apparatus, more particularly to apparatus employing magnetic field modulation and a pulsed laser beam, capable of writing new information directly over old information.
Magneto-optical recording is a type of perpendicular magnetic recording in which a laser beam is focused onto a rotating disk in the presence of a magnetic field. Laser illumination raises a magnetic layer in the disk above its Curie temperature, thus demagnetizing it. When the magnetic layer leaves the beam spot and cools, it acquires the magnetic orientation of the magnetic field. Binary data 1 and 0 can be written by creating up- and down-oriented magnetic domains in circular or spiral tracks on the disk.
The extensive prior art in this field falls into two general categories: optical modulation and magnetic field modulation. In an optical modulation system, the laser beam is switched between high and low power levels according to the information to be recorded. To write new information over old, such a system either requires a disk with a complex multiple magnetic layer structure, or requires the disk to make two turns per track, the polarity of the magnetic field being switched between the two turns.
Magnetic field modulation is simpler: a single magnetic layer suffices and the disk need make only one turn per track. The laser beam can be held at a constant level while the polarity of the magnetic field is switched according to the information to be recorded.
A problem with magnetic field modulation is that the magnetic domains created on the disk have tails that degrade signal-to-noise performance. Use of a pulsed laser beam delivering one pulse per recorded bit has been proposed, but this scheme tends to leave gaps between recorded bits, especially near the perimeter of the disk, so that old information is incompletely erased, which again degrades signal-to-noise performance.